A device that utilizes a piezoelectric phenomenon is applied in a wide range of fields. The application of such a device is increasing particularly in portable apparatuses such as mobile phones for which size reduction and power consumption saving are strongly demanded. IF (Intermediate Frequency) and RF (Radio Frequency) filters are examples of the application of the device. Specific examples of the IF and RF filters include an SAW (Surface Acoustic Wave) filter that uses an elastic SAWR (Surface Acoustic Wave Resonator).
The SAW filter is a filter that uses a resonator utilizing acoustic waves that travels a surface of solid. As a result of improvement in design and production engineering, the SAW filter satisfies difficult requests from a user. However, as a frequency that the SAW filter utilizes becomes higher, the SAW filter approaches to a limit of improvement in characteristics.
Therefore, an FBAR (Film Bulk Acoustic Resonator) filter has been developed as a new filter that is to replace the SAW filter. The FBAR filter uses a thin FBAR that is one of RF-MEMS (Radio Frequency-Micro Electro Mechanical System) devices.
The RF-MEMS is a technique that has been spotlighted recently. In this technique, a mechanical fine structure is fabricated mainly on a semiconductor substrate and an MEMS is applied to an RF front end. The MEMS is a technique for fabricating a device such as fine actuator, sensor, oscillator or the like.
The FBAR filter that is one of RF-MEMS devices is a filter that uses a resonator employing a thickness vertical vibration mode of a thin film that exhibits a piezoelectric response. In other words, the FBAR filter is a filter that uses a resonator utilizing a phenomenon in which a piezoelectric thin film vibrates in a thickness vertical direction in response to an inputted high frequency electrical signal and this vibration produces resonance in a thickness direction of the thin film. In this filter, resonance in a gigahertz frequency range is possible. The FBAR filter that has a characteristic as mentioned above has low loss. Moreover, the FBAR filter can operate in a broad band and realizes further size reduction and power consumption saving of portable apparatuses.
Moreover, RF-MEMS devices other than the FBAR filter, for example, an RF-MEMS capacitor and an RF-MEMS switch also realize low loss, high isolation, and low distortion in a high frequency range, by utilizing a piezoelectric phenomenon.
Examples of a piezoelectric material that constitutes a piezoelectric thin film used for the aforesaid RF-MEMS devices or the like are aluminum nitride (AlN), zinc oxide (ZnO), lithium niobate (LiNbO3), and lead zirconate titanate (Pb (Zr, Ti) O3; PZT). In particular, a piezoelectric thin film including aluminum nitride has preferable characteristics of elastic wave propagation speed, Q value, and frequency-temperature coefficient. Accordingly, aluminum nitride is known as a very preferable piezoelectric material for a piezoelectric thin film resonator of a filter that operates in a high frequency range (See, for example, Japanese Unexamined Patent Publication No. 344279/2002 (Tokukai 2002-344279) (published on Nov. 29, 2002)).
Japanese Unexamined Patent Publication No. 344279/2002 discloses that addition of a third component such as alkaline earth metal and/or rare earth metal to an aluminum nitride thin film improves a resonance characteristic.
However, the aluminum nitride thin film has a piezoelectric constant that is lower than those of other piezoelectric materials. Specifically, a piezoelectric constant d33 of the aluminum nitride thin film is approximately 5.1 pC/N to 6.7 pC/N. On the other hand, a piezoelectric constant d33 of a zinc oxide thin film is approximately 9.9 pC/N to 12.4 pC/N and a piezoelectric constant d33 of a lithium niobate thin film is approximately 6 pC/N to 12 pC/N. Further, a piezoelectric constant d33 of a lead zirconate titanate thin film is approximately 97 pC/N to 100 pC/N. In other words, the piezoelectric constant of the aluminum nitride thin film is approximately ½ to 1/20 of the piezoelectric constant of each of thin films that are made of other piezoelectric materials.
Accordingly, for example, a case where a device such as an RF-MEMS device employs a piezoelectric thin film including an aluminum nitride thin film requires an operation voltage that is higher than that in a case employing other piezoelectric material such as zinc oxide. That is, in the device that includes the piezoelectric thin film containing aluminum nitride, for example, an RF-MEMS device, power consumption saving becomes difficult.
Further, due to the low piezoelectric constant, an actuator employing the piezoelectric thin film containing aluminum nitride has an operational range that is narrower than that of an actuator employing a piezoelectric thin film that includes a piezoelectric material such as zinc oxide having a high piezoelectric constant. Further, in case where the piezoelectric thin film containing aluminum nitride is employed in a filter, loss becomes larger. In other words, the low piezoelectric constant of aluminum nitride is one factor that prevents size reduction and improvement of performance of a device employing the piezoelectric thin film containing aluminum nitride.
The present invention is attained in view of the aforesaid problems. The main object of the present invention is to provide a piezoelectric thin film including an aluminum nitride thin film.